Table 1. The attributes we used to describe the habitat associations of red-listed epiphytic lichens in Finland, the sources of information for the attributes, their categorization and the method of assigning the categories to lichen species.
Source of information Attribute Categories Method
Species records Primary host tree species Finnish tree species Tree species with the most records of the lichen species
Secondary host tree species Finnish tree species Up to three tree species with a considerable amount of records of the lichen species
Substrate Corticolous, Lignicolous, Lichenicolous The substrate with the most records of the lichen species
Distribution Hemiboreal, Southern, Northern,
Wide latitudinal range 		See text
Expert
assessments 		Habitat type The classification in the Finnish red list (Hyvärinen et al. 2019) Preferred conditions
Forest age Young (<60 yrs), Mature (60–120 yrs),
Old (>120 yrs), Indifferent 		Minimum age class required by the lichen species
Tree age Young (<60 yrs), Mature (60–120 yrs),
Old (>120 yrs) 		Minimum age class required by the lichen species
Humidity Dry, Intermediate, Humid Preferred conditions
Light availability Low, Intermediate, High Preferred conditions
1

Fig. 1. The number of primarily (black bars) and secondarily (grey bars) hosted red-listed epiphytic lichen species hosted by different tree species in Finland. The species that have undergone a genuine negative threat category change are represented by the white bars. The white bars (species that have undergone a negative category change) are a subset of the black bars (primarily hosted species). Primary host could not be assigned to 21 species (see Methods section).

Fig. 2. The relation of the number of red-listed epiphytic lichens hosted primarily or secondarily by a tree species to the total volume of the tree species in Finland (total volumes of tree species from NFI data, Korhonen et al. 2021a). Note that the x-axis is logarithmic. The regression line represents the expected number of primarily hosted species based on the total volume of a tree species.

Table 2. The residuals (i.e. the distance from the regression line) of each tree species in Fig. 2. The residuals originate from a linear regression where the number of red-listed epiphytic lichens hosted primarily or secondarily by a tree species was the dependent variable, and the total volume of the tree species in Finland was the independent variable. The regression line (i.e. residual value 0) represents the expected number of primarily hosted species based on the total volume of a tree species, and positive residuals indicate that the tree species hosts more red-listed epiphytic lichen species than expected from its total volume in Finland, while negative residuals indicate the opposite.
Tree species Residual
Populus 20.70
Picea 18.95
Sorbus 14.23
Quercus 5.64
Ulmus 5.52
Salix 5.07
Fraxinus 4.09
Acer 0.35
Corylus –1.88
Betula –2.26
Tilia –2.94
Alnus incana –14.01
Juniperus –15.11
Pinus –15.51
Alnus glutinosa –22.84
Table 3. The number of red-listed epiphytic lichens and epiphytic lichen species that have undergone a genuine negative threat category change in Finland by a) primary substrate type, b) primary deadwood type (lignicolous species), c) geographical distribution. One species had resin as its substrate, and it is not included in a).
  Number of species % No. of category change species %
a) Substrate        
      Corticolous 163 70.6 56 77.8
      Lignicolous 57 24.7 15 20.8
      Lichenicolous 10 4.3 1 1.4
b) Deadwood type        
      Downed 19 33.3 3 20.0
      Standing 28 49.1 10 66.7
      Wooden structures 9 15.8 2 13.3
      Unknown 1 1.8 0 0.0
c) Distribution        
      Hemiboreal 24 10.4 4 5.6
      Southern 71 30.7 17 23.6
      Northern 11 4.8 4 5.6
      Wide range 125 54.1 47 65.3
3

Fig. 3. The number of red-listed epiphytic lichen species by primary (black bars) and secondary (grey bars) habitat types.

4

Fig. 4. The number of red-listed epiphytic lichens by minimum age class requirement concerning forest age (black bars) and tree age (grey bars), and the number of species that have undergone a genuine negative threat category change (white bars) by forest and tree age class. Forest age and tree age were not applicable to two species (one of them had undergone a category change) that have been observed only on wooden structures in Finland.

Table 4. The number of a) red-listed epiphytic lichens, b) species that have undergone a genuine negative threat category change, with various microclimatic preferences concerning light availability (vertical) and humidity (horizontal).
a) Dry Intermediate Humid Unknown
High 9 33 24 6 72
Intermediate 0 82 47 3 132
Low 0 7 13 0 20
Unknown 0 0 3 4 7
9 122 87 13
b) Dry Intermediate Humid Unknown
High 2 11 4 0 16
Intermediate 0 39 10 1 48
Low 0 2 3 0 5
Unknown 0 0 0 0 0
2 50 17 1